Fabrication and gas-sensing properties of hierarchically porous ZnO architectures

Hierarchically three-dimensional (3D) porous ZnO architectures are synthesized by a template-free, economical aqueous solution method combined with subsequent calcination. First, the precursors of interlaced and monodisperse basic zinc nitrate (BZN) nanosheets are prepared. Then calcination of the precursors produces hierarchically 3D porous ZnO architectures composed of interlaced ZnO nanosheets with high porosity resulting from the thermal decomposition of the precursors. The products are characterized by X-ray diffraction, thermogravimetric-differential thermalgravimetric analysis, scanning electron microscopy, transmission electron microscopy, and Brunauer-Emmett-Teller N₂ adsorption-desorption analyses. The BET surface area of the hierarchically porous ZnO nanostructures was calculated to be 12.8 m² g⁻¹. Compared with ZnO rods, the as-prepared porous ZnO nanosheets exhibit a good response and reversibility to some organic gases, such as ethanol and acetone. The responses to 100 ppm ethanol and acetone are 24.3 and 31.6, respectively, at a working temperature of 320 °C. These results show that the porous ZnO architectures are highly promising for gas sensor applications, as the gas diffusion and mass transportation in sensing materials are significantly enhanced by their unique structures. Moreover, it is believed that this solution-based approach can be extended to fabricate other porous metal oxide materials with a unique morphology or shape.     

Improving the ethanol sensing of ZnO nano-particle thin films—The correlation between the grain size and the sensing mechanism

ZnO and Sn doped ZnO (ZnO:Sn) thin films at various doping concentrations from 1 to 10 at.% were prepared by the sol-gel method for an ethanol sensing application. The Sn doping significantly influenced the film growth, grain size and response of the films. The XRD patterns showed that the hexagonal wurtzite structure of the ZnO film was retained even after the Sn doping. The crystallite grain sizes of the ZnO:Sn thin films at 0, 2 and 4 at.% were estimated by using the typical Scherrer's equation. The crystalline quality of the films at 6, 8 and 10 at.% of Sn was degenerated. Typical FESEM images demonstrated the different morphologies for the ZnO:Sn thin films at various Sn concentrations; many pores of various dimensions were observed depending on the doping level. A TEM analysis of the ZnO:Sn thin films at 0, 2 and 4 at.% was performed to verify the grain size. The optimum Sn doping level of ZnO:Sn thin film for ethanol sensing was estimated to be 4 at.%. The 4 at.% sample obtained the highest response to ethanol vapor in the 10-400 ppm level range at a low operating temperature of 250 °C. The sensing mechanism was explained by a variation in the sensitivity model from a neck-grain-boundary controlled sensitivity to a neck-controlled sensitivity. Our work demonstrates the ability to reduce the working temperature as well as to increase the response of ZnO thin film based gas sensors to detect ethanol, which would be of great merit for commercialized applications.     

CdO activated Sn-doped ZnO for highly sensitive, selective and stable formaldehyde sensor

Sn-, Ni-, Fe- and Al-doped ZnO and pure ZnO are prepared by coprecipitation method, and characterized by scanning electron microscope (SEM), energy diffraction spectra (EDS) and X-ray diffraction (XRD). Their formaldehyde gas sensing properties are evaluated and the results show that 2.2 mol% Sn dopant can increase the response of ZnO by more than 2 folds, while other dopants increase little response or even decrease response. Further, CdO is used to activate ZnO based formaldehyde sensing material. It is demonstrated that 10 mol% CdO activated 2.2 mol% Sn-doped ZnO has the highest formaldehyde gas response, with a linear sensitivity of ∼10/ppm at lowered work temperature of 200 °C than 400 °C of pure ZnO, and high selectivity over toluene, CO and NH₃, as well as good stability tested in 1 month.     

High-performance liquefied petroleum gas sensing based on nanostructures of zinc oxide and zinc stannate

Toxic and combustible gas detection plays a major role in environmental air quality monitoring. Real-time monitoring of hazardous gases and signal of accidental leakages is of great importance owing to the concern for safety requirements in industries and household applications. A simple and economical method for the fabrication of highly sensitive zinc oxide (ZnO) nanorods based gas sensors for detecting low concentrations of Liquefied Petroleum Gas (LPG) was studied in this work. Platinum (Pt) nanoparticles were deposited on the sensing medium which acts as catalysts to improve the sensor performance. The change in electrical resistance of the metal oxide semiconductor for varying concentrations of LPG was measured. Maximum response of 59% was achieved for 9000 ppm LPG at 250 °C. Further to improve the sensing performance of the sensor towards LPG, surface modification of ZnO nanorods using zinc stannate (Zn₂SnO₄) microcubes was performed. High response of 63% was observed for 3000 ppm LPG at 250 °C. Significant improvement in response of the sensor with Zn₂SnO₄ microcubes on ZnO nanorods was observed when compared to sensor with ZnO nanorods.     

Nanostructured spinel ZnCo2O4 for the detection of LPG

Nanostrucutred spinel ZnCo₂O₄ (∼26-30 nm) was synthesized by calcining the mixed precursor (consisting of cobalt hydroxyl carbonate and zinc hydroxyl carbonate) in air at 600 °C for 5 h. The mixed precursor was prepared through a low cost and simple co-precipitation/digestion method. The transformation of the mixed precursor into nanostructured spinel ZnCo₂O₄ upon calcinations was confirmed by X-ray diffraction (XRD) measurement, thermogravimetric analysis (TGA), X-ray photoelectron spectroscopy (XPS) and high resolution transmission electron microscopy (HRTEM). To demonstrate the potential applicability of ZnCo₂O₄ spinel in the fabrication of gas sensors, its LPG sensing characteristics were systematically investigated. The ZnCo₂O₄ spinel exhibited outstanding gas sensing characteristics such as, higher gas response (∼72-50 ppm LPG gas at 350 °C), response time (∼85-90 s), recovery time (∼75-80 s), excellent repeatability, good selectivity and relatively lower operating temperature (∼350 °C). The experimental results demonstrated that the nanostructured spinel ZnCo₂O₄ is a very promising material for the fabrication of LPG sensors with good sensing characteristics. Plausible LPG sensing mechanism is also discussed.     

Gas sensing properties of a clad modified fiber optic sensor with Ce, Li and Al doped nanocrystalline zinc oxides

Spectral characteristics of a clad modified fiber optic gas sensor are studied for various concentrations (0-500 ppm) of ammonia, methanol and ethanol at room temperature. Cerium, aluminum and lithium doped (6 at.%) nanocrystalline zinc oxides are replaced with a clad and used as gas sensing materials. The study shows that the spectral intensity increases linearly with concentration for ammonia whereas it decreases for methanol and ethanol. The Ce doped ZnO exhibits higher gas sensitivity compared to Al and Li doped zinc oxides. The time response of the sensor is presented for a Ce doped ZnO with ammonia gas. A model is proposed for understanding the spectral intensity variations.     

Fabrication of gas sensing devices with ZnO nanostructure by the low-temperature oxidation of zinc particles

A method for low-temperature synthesis of a mixture of high-density ZnO nanoflakes and nanowires was developed to produce low-cost and high-efficiency gas sensors with ZnO nanostructures. ZnO nanoflakes and nanowires were grown on glass substrates by the RF sputter deposition of Zn particles and localized oxidation at a low temperature of 300 °C. The synthesized ZnO nanoflakes and nanowires were polycrystalline and had nanometer dimensions, as revealed by X-ray diffraction (XRD) and field emission scanning electron microscope (FESEM) measuring. A gas sensor based on the mixture of ZnO nanoflakes/nanowires responded rapidly and sensitively to ethanol. The sensing properties of the ZnO nanostructure sensor were approximately 72% for 50 ppm ethanol gas at an operating temperature of 100 °C. The response to 10 ppm of ethanol gas was 42% at the same temperature.     

Fabrication of N-type Fe2O3 and P-type LaFeO3 nanobelts by electrospinning and determination of gas-sensing properties

N-type Fe₂O₃ nanobelts and P-type LaFeO₃ nanobelts were prepared by electrospinning. The structure and micro-morphology of the materials were characterized by X-ray diffraction (XRD) and scanning of electron microscopy (SEM). The gas sensing properties of the materials were investigated. The results show that the optimum operating temperature of the gas sensors fabricated from Fe₂O₃ nanobelts is 285 °C, whereas that from LaFeO₃ nanobelts is 170 °C. Under optimum operating temperatures at 500 ppm ethanol, the response of the gas sensors based on these two materials is 4.9 and 8.9, respectively. The response of LaFeO₃-based gas sensors behaves linearly with the ethanol concentration at 10-200 ppm. Sensitivities to different gases were examined, and the results show that LaFeO₃ nanobelts exhibit good selectivity to ethanol, making them promising candidates as practical detectors of ethanol.     

Gas sensing properties at room temperature of a quartz crystal microbalance coated with ZnO nanorods

Gas sensors based on a quartz crystal microbalance (QCM) coated with ZnO nanorods were developed for detection of NH₃ at room temperature. Vertically well-aligned ZnO nanorods were synthesized by a novel wet chemical route at a low temperature of 90 °C, which was used to grow the ZnO nanorods directly on the QCM for the gas sensor application. The morphology of the ZnO nanorods was examined by field-emission scanning electron microscopy (FE-SEM). The diameter and length of the nanorods were 100 nm and 3 μm, respectively. The QCM coated with the ZnO nanorods gas sensor showed excellent performance to NH₃ gas. The frequency shift (Δf) to 50 ppm NH₃ at room temperature was about 9.1 Hz. It was found that the response and recovery times were varied with the ammonia concentration. The fabricated gas sensors showed good reproducibility and high stability. Moreover, the sensor showed a high selectivity to ammoniac gas over liquefied petroleum gas (LPG), nitrous oxide (N₂O), carbon monoxide (CO), nitrogen dioxide (NO₂), and carbon dioxide (CO₂).     

LPG sensing properties of Pd-sensitized vertically aligned ZnO nanorods

One-dimensional (1-D) vertically aligned ZnO nanorods are synthesized on glass substrate through a simple chemical route and their liquefied petroleum gas (LPG) sensing properties are studied. The morphology and structure of vertically aligned ZnO nanorods has been characterized by scanning electron microscopy (SEM) and X-ray diffraction (XRD) analysis. The LPG sensing properties of the vertically aligned ZnO nanorods are improved significantly after palladium (Pd) sensitization. The unsensitized vertically aligned ZnO nanorods exhibited the maximum response of 37% at 573 K upon exposure to 2600 ppm LPG, which improved to 60% at operating temperature of 498 K after the Pd sensitization. The Pd-sensitized vertically aligned ZnO nanorods showed more selectivity towards LPG as compared to CO₂. Our results demonstrate that the chemically grown vertically aligned ZnO nanorods along with Pd sensitization are promising material for the fabrication of cost effective and high performance gas sensors.     

Enhanced ammonia sensing performances of Pd-sensitized flowerlike ZnO nanostructure

We report the synthesis of flowerlike ZnO nanostructure using a facile hydrothermal process, and the investigation on the ammonia (NH₃)-sensing properties of the pure and palladium (Pd)-sensitized flowerlike ZnO nanostructure. The phase purity, morphology, and structure of the pure and Pd-sensitized ZnO nanostructure are investigated. The characterized results reveal that the flowerlike ZnO has a wurtzite structure and is composed of numerous aggregated single-crystalline ZnO nanorods with a diameter of about 60 nm. Having fabricated gas sensors based on the pure and Pd-sensitized flowerlike ZnO, we find that the Pd-sensitized sensor exhibits a response of 45.7-50 ppm NH₃ at 210 °C, which is about 8 times higher than that of pure ZnO at the optimal operating temperature of 350 °C. The enhanced NH₃-sensing performance demonstrates that the significant decrease in optimal operating temperature and the distinct increase in response are attributed to the sensitization effect of Pd.     

Improved selective acetone sensing properties of Co-doped ZnO nanofibers by electrospinning

Pure and Co-doped (0.3 wt%, 0.5 wt%, and 1 wt%) ZnO nanofibers are synthesized by an electrospinning method and followed by calcination. The as-synthesized nanofibers are characterized by X-ray powder diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and energy dispersive X-ray (EDX) spectroscopy. Comparing with pure ZnO nanofibers, Co-doped nanofibers exhibit improved acetone sensing properties at 360 °C. The response of 0.5 wt% Co-doped ZnO nanofibers to 100 ppm acetone is about 16, which is 3.5 times larger than that of pure nanofibers (about 4.4). The response and recovery times of 0.5 wt% Co-doped ZnO nanofibers to 100 ppm acetone are about 6 and 4 s, respectively. Moreover, Co-doped ZnO nanofibers can successfully distinguish acetone and ethanol/methanol, even in a complicated ambience. The high response and quick response/recovery are based on the one-dimensional nanostructure of ZnO nanofibers combining with the Co-doping effect. The selectivity is explained by the different optimized operating temperatures of Co-doped ZnO nanofibers to different gases.     

Self-heated hydrogen gas sensors based on Pt-coated W18O49 nanowire networks with high sensitivity, good selectivity and low power consumption

Self-heating effect can be applied to develop gas sensors avoiding use of external heaters. We report here a prototype self-heated hydrogen gas sensor based on Pt-coated W₁₈O₄₉ nanowire networks. Such a sensor has been shown to have promising performance, e.g., high sensitivity (able to detect down to 50 ppm of H₂), good selectivity (poor response to ethanol, CH₄, CO, and C₃H₈), and low power consumption of 30-60 mW at 6 V compatible to a portable device. Coaxial cable model and percolation theory are applied to explain the gas sensing effect of the Pt-coated W₁₈O₄₉ nanowire networks.     

Acetylene sensor based on Pt/ZnO thick films as prepared by flame spray pyrolysis

ZnO nanoparticles loaded with 0.2-2.0 at.% Pt have been successfully produced in a single step by flame spray pyrolysis (FSP) technique using zinc naphthenate and platinum(II) acetylacetonate, as precursors dissolved in xylene and their acetylene sensing characteristics have been investigated. The particle properties were analyzed by XRD, BET, TEM, SEM and EDS. Under the 5/5 (precursor/oxygen) flame condition, ZnO nanoparticles and nanorods were observed. The crystallite sizes of ZnO spherical and hexagonal particles were found to be ranging from 5 to 20 nm while ZnO nanorods were seen to be 5-20 nm in width and 20-40 nm in length. In addition, very fine Pt nanoparticles with diameter of ∼1 nm were uniformly deposited on the surface of ZnO particles. From gas-sensing characterization, acetylene sensing characteristics of ZnO nanoparticles is significantly improved as Pt content increased from 0 to 2  at.%. The 2 at.% Pt loaded ZnO sensing film showed an optimum C₂H₂ response of ∼836 at 1% acetylene concentration and 300 °C operating temperature. A low detection limit of 50 ppm was obtained at 300 °C operating temperature. In addition, Pt loaded ZnO sensing films exhibited good selectivity towards hydrogen, methane and carbon monoxide.     

Highly sensitive and selective LPG sensor based on α-Fe2O3 nanorods

The α-Fe₂O₃ nanorods were successfully synthesized without any templates by calcining the α-FeOOH precursor in air at 300 °C for 2 h and their LPG sensing characteristics were investigated. The α-FeOOH precursor was prepared through a simple and low cost wet chemical route at low temperature (40 °C) using FeSO₄·7H₂O and CH₃COONa as starting materials. The formation of α-FeOOH precursor and its topotactic transformation to α-Fe₂O₃ upon calcination was confirmed by X-ray diffraction measurement (XRD), X-ray photoelectron spectroscopy (XPS), field emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM) analysis. The α-Fe₂O₃ nanorods exhibited outstanding gas sensing characteristics such as, higher gas response (∼1746-50 ppm LPG at 300 °C), extremely rapid response (∼3-4 s), relatively slow recovery (∼8-9 min), excellent repeatability, good selectivity and lower operating temperature (∼300 °C). Furthermore, the α-Fe₂O₃ nanorods are able to detect up to 5 ppm for LPG with reasonable response (∼15) at the operating temperature of 300 °C and they can be reliably used to monitor the concentration of LPG over the range (5-60 ppm). The experimental results clearly demonstrate the potential of using the α-Fe₂O₃ nanorods as sensing material in the fabrication of LPG sensors. Plausible LP G sensing mechanism of the α-Fe₂O₃ nanorods is also discussed.     

Love wave hydrogen sensors based on ZnO nanorod film/36°YX-LiTaO3 substrate structures operated at room temperature

Love wave hydrogen sensors based on ZnO nanorod layers deposited on 36°YX-LiTaO₃ substrates have been studied. The ZnO nanorod layers are prepared by two steps: first, the seed layers, as well the guiding layers of the Love wave devices, are deposited by RF magnetron sputtering; second, the nanostructural layers, as well the sensing layers of the sensors, are grown by hydrothermal synthesis. Two kinds of ZnO layers have been analyzed by XRD, SEM and XPS. The XRD shows that both ZnO layers have (0 0 2) oriented wurtzite structures. The SEM results reveal that the morphologies of the deposited ZnO seed layers are continuous and compact, while the hydrothermal treated layers are with nanorods almost perpendicular to the substrate surfaces. Finally, the hydrogen sensing responses of the Love wave sensors activated by Pt catalysts are measured for various concentrations of hydrogen in synthetic air at room temperature. The results show that the sensors have high sensitivity and repeatability as the nanorod layers are optimized, such as the frequency shift 8 kHz toward 0.04% of H₂ in synthetic air is obtained while the height of the nanorod layer is about 2.1 μm and the central frequency of the sensor is about 125.5 MHz. The XPS analyses of the sensitive layers show that there are oxygen vacancies in the layers, so the oxygen vacancy model is used to explain the hydrogen sensing mechanism of the Love wave sensors.     

Effect of aluminium doping on structural and gas sensing properties of zinc oxide thin films deposited by spray pyrolysis

A facile spray pyrolysis route is used to deposit aluminium doped ZnO (AZO) thin films on to the glass substrates. It is observed that on aluminium doping the particle size of ZnO reduces significantly; moreover, uniformity of particle also gets enhanced. Their XRD study reveals that intensity ratio of crystal planes depend on the aluminium doping concentration. The gas response studies of; ∼800 nm thick Al-doped ZnO films at different operating temperatures show that 5 at% Al-doped ZnO thin film exhibits highest response towards H₂S gas at 200 °C. The results suggest that the gas response strongly depends on the particle size and aluminium doping in the ZnO.     

Electrospun Cu-doped ZnO nanofibers for H2S sensing

Pure and Cu-doped ZnO nanofibers were synthesized via electrospinning technology. The morphology and structure of the as-synthesized nanofibers were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD) and Raman spectroscopy. The effects of Cu doping on H₂S sensing properties at low concentration (1-10 ppm) were investigated at 230 °C. The results show that the H₂S sensing properties of ZnO nanofibers are effectively improved by Cu doping: 6 at% Cu-doped ZnO nanofibers show a maximum sensitivity to H₂S gas, and the response to 10 ppm H₂S is one order of magnitude higher than the one of pure ZnO nanofibers.     

SnO2 thin film sensor with enhanced response for NO2 gas at lower temperatures

Semiconducting SnO₂ thin films having higher value of electrical conductivity have been deposited using RF sputtering technique in the reactive gas environment (30% O₂ + 70% Ar) using a metallic tin (Sn) target for detection of oxidizing NO₂ gas. The effect of growth pressure (12-18 mTorr) on the surface morphology and structural property of SnO₂ film was studied using Atomic force microscopy (AFM), Scanning electron microscopy (SEM) and X-ray Diffraction (XRD) respectively. Film deposited at 16 mTorr sputtering pressure was porous with rough microstructure and exhibits high sensor response (∼2.9 × 10⁴) towards 50 ppm NO₂ gas at a comparatively low operating temperature (∼100 °C). The sensor response was found to increase linearly from 1.31 × 10² to 2.9 × 10⁴ while the response time decrease from 12.4 to 1.6 min with increase in the concentration of NO₂ gas from 1 to 50 ppm. The reaction kinetics of target NO₂ gas on the surface of SnO₂ thin film at the Sn sites play important role in enhancing the response characteristics at lower operating temperature (∼100 °C). The results obtained in the present study are encouraging for realization of SnO₂ thin film based sensor for efficient detection of NO₂ gas with low power consumption.     

Selectivity of flame-spray-made Nb/ZnO thick films towards NO2 gas

Unloaded ZnO and Nb/ZnO nanoparticles containing 0.25, 0.5 and 1 mol.% Nb were produced in a single step by flame-spray pyrolysis (FSP) technique. The nanoparticles were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The BET surface area (SSA_BET) of the nanoparticles was measured by nitrogen adsorption. FSP yielded small Nb particles attached to the surface of the supporting ZnO nanoparticles, indicating a high SSA_BET. The morphology and accurate size of the primary particles were further investigated by TEM. Nb/ZnO nanoparticles paste composed of ethyl cellulose and terpineol as binder and solvent respectively was coated on Al₂O₃ substrate interdigitated with gold electrodes to form thick films by spin coating technique. After the sensing tests, the morphology and the cross-section of sensing film were analyzed by SEM and EDS analyses. The influence on a low dynamic range of Nb concentration on NO₂ response (0.1-4 ppm) of thick film sensor elements was studied at the operating temperatures ranging from 250 to 350 °C in the presence of dry air. The optimum Nb concentration was found be 0.5 mol.% and 0.5 mol.% Nb exhibited an optimum NO₂ response of ∼1640 and a short response time (27 s) for NO₂ concentration of 4 ppm at 300 °C.